Malignant melanoma is one of the most aggressive types of cancers. Its ability to metastasize and resistance to anticancer chemotherapy makes melanoma extremely difficult to cure. Additionally, there are no reliable molecular markers associated with melanoma progression. Invasion as a prerequisite for metastasis, is one of the most detrimental features of melanoma. Recently, using cultured cells, experimental animals and analysis of human melanoma specimens, we have identified guanosine monophosphate reductase (GMPR) as a suppressor of melanoma invasion. Our preliminary data show that functional antagonist of GMPR, guanosine monophosphate synthase (GMPS), is overexpressed in human metastatic melanomas and is required for melanoma cell invasion and growth of human melanoma xenografts in nude mice. Furthermore, we demonstrated that a relatively understudied chemical inhibitor of GMPS, decoyinine, reduces melanoma cell invasion. Moreover, our analysis of publically available databases identified several other guanylate metabolism enzymes that differentially express in primary and metastatic melanoma specimens. C-MYC is a transcription factor that regulates expression of multiple genes involved in promotion of proliferation, and augmentation of cellular metabolism, including biosynthesis of nucleotides. Up-regulation of C-MYC levels is one of the most frequent events in human malignancies including metastatic melanoma. Preliminary data in our laboratory suggested that GMPR is a C-MYC-responsive gene and that long-term GMPR suppression is mediated by epigenetic regulation. Therefore our central hypothesis is that progression to metastatic melanoma is facilitated by distortion of guanylate pools through disruption of normal biosynthetic pathways at multiple points. Our long term goal is to develop novel therapeutic approaches to treat or prevent metastatic melanoma, as well as to identify a set of biomarkers for early diagnosis of metastatic potential. Our objectives in the current proposal are i) to investigate the therapeutic potential of pharmacological inhibition of GMPS; ii) to characterize several rate- limiting enzymes involved in guanylate metabolism as potential melanoma markers and iii) to identify mechanisms underlying transcriptional suppression of GMPR in melanoma cells. To achieve these goals we propose the following three specific aims: 1) To determine the efficacy of the GMPS inhibitor decoyinine against melanoma xenografts in immunocompromised mice 2) To determine the prognostic value of enzymes involved in guanylate metabolism for overall survival (OS) of primary melanoma patients. 3)To investigate the mechanisms of GMPR regulation and the role of GMPR in MYC-dependent up-regulation of invasion. Melanoma is one of the incurable types of human cancers with no reliable prognostic molecular markers. Our proposed research will characterize a promising new target for melanoma therapy, discover new mechanisms underlying melanoma invasion, and identify new prognostic factors for melanoma patients.